The horizontal stabilizer of an aircraft is mainly comprised of two torsion boxes, joined to each other along the symmetry plane of an aircraft (configuration with two lateral torsion boxes) or along their interface with the fuselage (configuration with a center torsion box, aside from the lateral torsion boxes). Said torsion boxes are in turn constituted of different elements such as skins reinforced by longitudinal stringers, spars and ribs.
Nowadays and particularly in the aeronautical industry, composite materials having an organic matrix and continuous fibres, particularly an epoxy matrix and carbon fibre are massively used in a large variety of structural elements. For example, all the elements comprised in the above-mentioned torsion boxes (ribs, spars, skins, etc.) are currently manufactured using said composite materials, which will de referred to hereinafter as “CFRP” (Carbon fiber reinforced plastic) which are their English initials.
Composite materials are divided into those that, as a raw material before being processed, contain the resin pre-impregnating the carbon fibers; and those wherein the carbon fiber is not impregnated with resin (commonly known as dry fibers) and said resin is injected during the manufacturing process itself. Among these dry fabric materials is a non crimp fabric which, as it possesses good mechanic properties, can reproduce complex geometries.
In the joint between the torsion boxes it is necessary to ensure the load transmission between the elements involved, by means of a series of reinforcing/joining elements that ensure the structural integrity of the area, given that the horizontal stabilizer must act as a large structural unity. This joint has to withstand both the aerodynamic loads as well as loads derived from the mechanism used for moving or trimming the horizontal stabilizer.
In the specific case of the skins, these elements involved are two fittings with a triform section that not only join the top and bottom skins to each other, but are also joined to each other through a flat center rib. By means of these fittings that are assembled on the outside of the torsion boxes, a structural single shear joint between the skins is defined, balancing the loads in the horizontal direction and absorbing the vertical resultant load by means of the cited center rib.
These fittings have a very significant structural importance, their weight and cost being very high. In order to reduce the weight of the fittings as much as possible, as well as improving their performance against stress or corrosion, these fittings are generally made from titanium. The manufacturing process of these fittings implies a forging process and afterwards a numeric control machining process in order to provide them with the greatest strength in the main load direction and to ensure the tight tolerances required for the assembly thereof. Due to all of these factors the cost of these parts is very high.
Furthermore, it should be emphasized that the drilling of holes in this mechanic joint where, later on, the rivets that transmit the loads from some elements to others will be installed, are very complex due to the fact that it is a hybrid junction between metallic elements (fittings) and elements made from non-metallic composite materials (CFRP). These non-metallic elements are the skins (cover) including the feet of the stringers (longitudinal stiffeners elements). This complexity, mainly due to the presence of metal elements, implies that said drilling operations are long and costly.